Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. Large format disks, and more recently, DVD disks, have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD-ROM, have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, are encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data are read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the field of laser illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal that is decoded to recover the digital data stored on the optical disk.
To retrieve data from an optical disk, the optical system includes a pickup assembly that may be positioned to read data on any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity. Additional detail on previously known optical disk readers may be found in H. Nakajima and H. Ogawa, Compact Disc Technoloay, (translated by C. Aschmann), published by Ohmsha, Ltd., Japan (1992), and in K. Pohlmann, The Compact Disc Handbook, (2nd ed. 1992), published by A-R Editions, Inc., Madison, Wis.
Because in most previously known systems the data are read from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 .mu.m, while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, constant linear velocity (CLV) drives with rotational speeds of up to 16.times. standard speed are commercially available, and even faster reading speeds have been achieved using constant angular velocity designs. Higher disk rotational speeds, however, place increased demands on the optical and mechanical subsystems within the optical disk player, create greater vibration, and may make such players more difficult and expensive to design and manufacture.
A cost effective alternative to increasing the disk rotational speed is to read multiple data tracks simultaneously. One such implementation uses multiple beams, arranged so that each beam illuminates a single data track on a disk. U.S. Pat. No. 5,144,616 to Yasukawa et al. shows a system in which multiple laser diode emitters are used to provide multiple beams. Other methods may also be used to provide multiple beams, though some of these methods may not be appropriate for use in writing multiple tracks simultaneously. For example, U.S. Pat. No. 4,459,690 to Corsover describes a multi-beam system in which an illumination beam generated by a single laser source is split into multiple beams using an acousto-optic device that dithers the beam in a direction normal to the track direction.
The beams in a multi-beam optical pickup may also be provided by using a diffractive element to split a single beam into multiple beams. This technique is used to generate the beams in a three-beam tracking system, as shown in The Compact Disc Handbook, Pohlmann, K., 2nd ed., A-R Editions, 1992, pp. 108-115. In commonly assigned, copending U.S. patent application Ser. No. 08/911,815, a diffractive element is used to split an illumination beam into a plurality of reading beams. Through careful design, it is possible to produce a diffractive element capable of generating multiple reading beams properly aligned with the data tracks of an optical disk.
One difficulty encountered by optical disk readers that read multiple tracks of an optical disk simultaneously is delivering data to the host computer in sequential order. When reading a typical optical disk, each of the multiple beams in a multi-beam system reads a sequence of blocks of data. The blocks read by each beam are not sequential with the blocks read by other beams. For example, a first beam of a multi-beam optical disk reader may read data block 100 of an optical disk, while a second beam is reading data block 126, and a third beam is reading block 157. Thus, data blocks are read from the disk in a non-sequential order.
Moreover, it is necessary to determine when all the data blocks of one set of tracks have been read, so that the optical pickup can be moved to read a new set of tracks. If this determination is inaccurate, many data blocks may need to be read more than once, decreasing the efficiency and performance of the optical disk reader. Also, when the optical pickup is moved, it is necessary to make sure that one of the tracks being read contains a desired data block.
The host computer that receives data from an optical disk reader typically requires that the data be delivered in sequential order. Thus, it is necessary for optical disk readers that read multiple tracks simultaneously to reorder the data blocks, so they may be delivered sequentially to the host computer.
It would therefore be desirable to provide methods and apparatus for buffering data that is simultaneously read from multiple tracks of an optical disk, and for reordering the data so it may be delivered to a host computer in sequential order.
It would further be desirable to provide methods and apparatus for determining when all of the data blocks in a set of tracks have been read, permitting the optical pickup to be moved to read a new set of tracks.
Additionally, it would be desirable to provide methods and apparatus for determining whether a set of tracks of an optical disk contains a desired data block.